Circuit arrangement for operating low-pressure discharge lamps

ABSTRACT

A circuit arrangement for high-frequency operation of a low-pressure  discge lamp (18) includes a mains rectifier (10), a radio interference suppression filter (12) connected to the mains rectifier (10), an RF inverter (14), connected to the direct current output of the mains rectifier (10) and having two alternatingly switching transistors (T 1 , T 2 ) along with an inductance (L K ), a trigger circuit and a center tap (M), which is between the two transistors (T 1 , T 2 ). A filter capacitor (C S ) is connected parallel to the switching paths of the two transistors (T 1 , T 2 ) of the RF inverter (14). A series resonant circuit (16) is assigned to the low-pressure discharge lamp (18), and comprises a resonant inductance (L 3 ), a coupling capacitor (C 3 ), and a resonant capacitance parallel to the lamp. The connection lines for the low-pressure discharge lamp (18) lead on the one hand from a first electrode (E 1 ) via the resonant inductance (L 3 ) to the center tap (M) and on the other from a second electrode (E 2 ) via the center tap of a diode series circuit, connected in the direct current forward direction in series with the filter capacitor (C S ) and comprising a first and a second diode (D 1 , D 2 ), to the positive and negative terminal, respectively, of the mains rectifier (10). The capacitance parallel to the lamp of the series resonant circuit (16) assigned to the low-pressure discharge lamp (18) comprises two parallel-connected capacitors (C R1 , C R2 ); one (C R2 ) of the capacitors (C R1 , C R2 ) parallel to the lamp is connected directly to the positive or negative terminal of the filter capacitor (C S ).

FIELD OF THE INVENTION

The-invention relates to a circuit arrangement for high-frequency operation of one or more parallel- or serially-connected low-pressure discharge lamps.

BACKGROUND

A circuit arrangement to operate low-pressure discharge lamps is described in European Patent Application 0 395 776, Krummel. However, it has the disadvantage, first, that higher costs and a greater space requirement are created as a result of additional limiter diodes at the lamp base point, along with a necessarily fast bridge rectifier and an expensive radio interference suppressor filter on the alternating voltage side. Secondly, the circuit is also less suitable for relatively low mains voltages combined with high lamp burning and igniting voltages, since the full resonant capacitance is parallel to the lamp and consequently the converted idle output--which is necessary to generate the high burning and igniting voltages--undergoes a very major modulation as a function of the instantaneous value of the rectified mains voltage.

THE INVENTION

It is an object of the present invention to provide a circuit arrangement of the type described at the outset, which, with technically simple means, without increased interfering radiation and at low additional power losses, assures both an increase in the power factor of the circuit arrangement and reliable operation without a protective circuit.

The circuit according to the invention includes a pump circuit, in which the positive terminal of the rectified alternating mains voltage, on the output side of a radio interference suppression filter, is connected to the base point of the lamp via a first fast diode polarized in the forward direction. This point is connected to the positive terminal of the filter capacitor via a second diode also polarized in the forward direction.

The modulation of the lamp current and the generation of the harmonics is reduced by the capacitor being located parallel to the second diode of the diode series circuit.

Further radio interference suppression is achieved by means of a further capacitor, which is connected parallel to the first diode.

Preferably, the capacitance of the series resonant circuit assigned to each low-pressure discharge lamp is formed by two parallel-connected capacitors; the following inequality is desirable for the ratio between the capacitances of the two capacitors C_(R1) and C_(R2) (connected to the filter capacitor):

    0.5<C.sub.R1 /C.sub.R2 <2

In a further embodiment, the other of the two capacitors, given suitable filaments, can be replaced by suitably modified preheating capacitors.

According to the invention, a current takeup from the mains during one-half of a period of the RF inverter results, and then charging (pumping) of the filter capacitor during the other half period. By placing a choke before the first diode and connecting a capacitor parallel to the second diode, it is assured that the charging process will not be interrupted at the time periods when the mains voltage drops below half the filter capacitor voltage. Overpumping or a voltage overload especially during ignition is prevented in this circuit arrangement due to attenuation of the pump process, by providing that the resonant capacitance parallel to the lamp is split into at least two capacitors, and one of the at least two capacitors is connected directly to the positive or negative terminal, respectively, of the filter capacitor. This not only prevents an overvoltage at this capacitor but also reduces the modulation of the lamp current, without also markedly worsening the ignitability of the circuit.

A further improvement is obtained by connecting a capacitor parallel to the first diode. The current across this first diode is very rich in harmonics and causes major radio interference, especially in the range above 1 MHz. The capacitor having a capacitance of approximately 1 to 5 nF brings about a pronounced reduction in harmonics of the operating frequency (50 kHz) and thus a drastic reduction in radio interference.

DRAWINGS

Further advantages and characteristics of the invention will become apparent from the ensuing description of various embodiments and from the drawings, to which reference is made. Shown are:

FIG. 1, a block circuit diagram of the circuit arrangement in a first embodiment;

FIG. 2, a circuit diagram of the circuit arrangement of FIG. 1;

FIG. 3, a block circuit diagram of a further embodiment of the circuit arrangement for operating a plurality of parallel-connected low-pressure discharge lamps with a common pump branch;

FIG. 4, a block circuit diagram of a further embodiment of the circuit arrangement for operating a plurality of parallel-connected low-pressure discharge lamps with separate pump branches;

FIG. 5, a block circuit diagram of a further embodiment of the circuit arrangement for operating a plurality of serially-connected low-pressure discharge lamps with a common pump branch.

DETAILED DESCRIPTION

In the block circuit diagram shown in FIG. 1, a high-frequency filter or radio interference suppression filter 12 is connected to the outputs of a mains rectifier 10, or vice versa, and that filter 12 is followed by an RF inverter 14 with two alternatingly switching transistors T₁ and T₂ and a trigger circuit with a center tap M (FIG. 2) between the two transistors T₁ and T₂. A low-pressure discharge lamp 18 is connected via a series resonant circuit, marked 16 in FIG. 2, between the center tap M of the two transistors T₁ and T₂ and the positive terminal of the radio interference suppression filter 12. A filter capacitor C_(S) connected between the two inputs of the RF inverter 14 is connected by its negative terminal to the negative terminal of the interference filter 12.

The positive terminal of capacitor C_(S) is connected to the positive terminal of the interference filter 12, via a series diode circuit comprising diodes D₁ and D₂ connected in series and in the direct current forward direction. An inductance L_(K) is located between the first diode D₁ of the diode series circuit and the positive terminal of the interference filter 12. It serves to charge the filter capacitor C_(S), when the mains voltage drops below half the C_(S) voltage. The first diode D₁ and the second diode D₂ provide that in one half-period of the RF inverter 14 the filter capacitor C_(S) is charged, and in the other half period current is taken from the mains. A capacitor C_(K) connected parallel to the diode D₂ assist in oscillation of the RF inverter 14 and improves the ignitability of the circuit. An additional capacitor C_(F), connected parallel to the first diode D₁, provides radio interference suppression. A first electrode E₁ of the low-pressure discharge lamp 18 is connected to the center tap M of the RF inverter via the inductance L₃ and a capacitor C₃ ; the second electrode E₂ of the low-pressure discharge lamp 18 is connected to a tap or base point W₁₁, which is located between the first diode D₁ and the second diode D₂. An ignition circuit 20 is connected to the respective other outputs of the electrodes E₁ and E₂.

In accordance with a feature of the invention, the resonance capacitance parallel to the lamp comprises two capacitors C_(R1) and C_(R2). The first capacitor C_(R1), via the base point W₁₁, connects together the two electrodes E₁ and E₂ of the low-pressure charge lamp 18 and is thus connected between diodes D₁ and D₂ of the series diode circuit.

The second capacitor C_(R2) is located parallel to the first capacitor C_(R1), but on the output side of the second diode D₂, and connected to one of the terminals, in FIGS. 1 and 2 to the positive terminal of filter capacitor C_(S).

In FIG. 2, a physical embodiment of the version of FIG. 1 is shown. In this circuit diagram of the circuit arrangement, the interference filter 12 is connected to the positive and negative terminal, respectively, of the mains rectifier 10; this filter comprises two capacitors Cπ₁ and Cπ₂, each with a capacitance of 150 nF, and an inductance Lπ of 680 μH, which connects the two capacitors Cπ₁ and Cπ₂.

The negative terminal of the filter capacitor C_(S) is connected via the interference filter 12 to the negative terminal of the mains rectifier 10, while the positive terminal of the filter capacitor C_(S) is connected, via the second diode D₂ and the capacitor C_(K) connected parallel to it, to the second electrode E₂ of the low-pressure discharge lamp 18, and via a fourth diode D4 to the interference filter 12. The second diode D₂ connects the filter capacitor C_(S) to the positive terminal of the mains rectifier 10, via the first diode D₁ and the choke L_(K) having an inductance of 470 μH, via the interference filter 12. The capacitor C_(F) connected parallel to the first diode D₁ has a capacitance of approximately 1 to 5 nF. The RF inverter 14 is on the one hand connected between the second diode D₂ and the positive terminal of the filter capacitor C_(S) and on the other is connected to the negative terminal of the mains rectifier 10 via the interference filter 12. The center tap M of the push-pull frequency generator 14 is connected to the first electrode E₁ of the low-pressure discharge lamp 18, via an inductance L₃ and a capacitor C₃. Together with the capacitors C₁ and C₂ parallel to the lamp, the inductance L₃ and the capacitor C₃ form the series resonant circuit 16 associated with the low-pressure discharge lamp 18.

The ignition circuit 20 connected to the other two terminals of the electrodes E₁ and E₂ has, parallel to the electrodes E₁ and E₂, two series-connected capacitors C₄ and C₅ ; a temperature-dependent resistor R₄ is provided parallel to the one capacitor C₄. The series circuit comprising the capacitors C₄ and C₅ contributes to the total resonant capacitance comprising C_(R1) and C_(R2).

FIGS. 3-5 show circuit arrangements with which two and more low-pressure discharge lamps 18, 18n can be operated.

In FIG. 3, a circuit arrangement is shown for operating two and more parallel-connected low-pressure discharge lamps 18 and 18n, all of which are operated with a common pump branch. One ignition circuit 20n each, one additional capacitor C_(R1n) and C_(R2n), which is connected parallel to the first capacitor C_(R1) and C_(R2), respectively, and one lamp choke L_(3n) each and one coupling capacitor C_(3n) each are provided for each further low- pressure discharge lamp 18n.

FIG. 4 shows a circuit arrangement for two and more parallel-connected low-pressure discharge lamps 18n with separate pump branches. The difference from the embodiment of FIG. 3 is that the further capacitor C_(R1n) is connected to a further pump branch. The further pump branch is connected parallel to the first pump branch and comprises the further inductance L_(Kn), a further diode series circuit D_(1n) and D_(2n), and the respective parallel-connected further capacitors C_(Kn) and C_(Fn).

One example of two and more low-pressure discharge lamps 18, 18n connected in series is shown in FIG. 5. All the series-connected low-pressure discharge lamps 18 and 18n have a single shared ignition circuit 20. The difference from the embodiment of FIG. 1 is that the first electrode E1 of the first low-pressure discharge lamp 18 is connected in series with the second electrode E_(2n) of the further low-pressure discharge lamp 18n and is connected to a galvanically separate preheater, which comprises an additional coil L_(3n), on the lamp choke L₃.

The following component list shows the circuit elements used for a circuit arrangement for operating a 20 W compact fluorescent lamp connected to 120 V alternating voltage:

    ______________________________________                                         C.sub.1 = 100 nF    R.sub.1 = 330 kΩ                                     C.sub.2 = 2.2 nF    R.sub.2 = 10 Ω                                       C.sub.3 = 150 nF    R.sub.3 = 470 kΩ                                     C.sub.4 = 2.2 nF                                                               C.sub.5 = 6.8 nF                                                               Cπ.sub.1 = Cπ.sub.2 = 150 nF                                             CR.sub.1 = 4.7 nF                                                              CR.sub.2 = 3.3 nF   Lπ = 680 μH                                          C.sub.S = 47 μF/33μF                                                                         L.sub.K = 330 μH                                        C.sub.F = 3.3 nF    L.sub.3 = 1.4 mH                                           C.sub.K = 15 nF                                                                D.sub.1 = BA 157    T.sub.1 = T.sub.2 = IRF 224                                D.sub.2 = BA 157                                                               D.sub.3 = 1N4004                                                               D.sub.4 = 1N4004                                                               ______________________________________                                     

I claim:
 1. A circuit arrangement for high-frequency operation of one or more low-pressure discharge lamps (18) connected to one another in parallel or in series, havinga mains rectifier (10); a radio interference suppression filter (12) connected to the mains rectifier (10); an RF inverter (14), connected to the direct current output of the mains rectifier (10) and having two alternatingly switching transistors (T₁ and T₂) , an inductance (L_(K)), a trigger circuit, and a center tap (M) between the two transistors (T_(l), T₂); a filter capacitor (C_(S)) parallel to the switching paths of the two transistors (T₁, T₂) of the RF inverter (14); one or more series resonant circuits (16), each assigned to one low-pressure discharge lamp (18), comprising a coupling capacitor (C₃), a resonant inductance (L₃), and a resonant capacitance parallel to the lamp; a series diode circuit comprising first and second diodes (D₁, D₂) connected in direct current forward direction, and defining a center point (W₁₁) therebetween, and having one input connected to said filter capacitor (C_(S)); connecting lines for the low-pressure discharge lamp or lamps (18), in which a first line of the first electrodes (E₁) of the low-pressure discharge lamp or lamps (18) is connected to the center tap (M) via the resonant inductance (L₃), and one further line, connected to each of the second electrode or electrodes (E₂) of the low-pressure discharge lamp or lamps (18) is further connected to the positive or negative terminal of the mains rectifier (10), via the center point (W₁₁) of the diode series circuit; characterized in that the resonant capacitance of the series resonant circuit (16) assigned to each low-pressure discharge lamp (18) is divided into a plurality of resonance capacitors (C_(R1), C_(R2)), each having one terminal connected together, one (CR₁) of the resonance capacitors (C_(R1), C_(R2)) is effectively connected across the electrodes of the lamp or lamps, and the other (C_(R2)) Of the capacitors (C_(R1), C_(R2)) has its other terminal connected directly to the positive or negative terminal of the filter capacitor (C_(S)).
 2. The circuit arrangement of claim 1 characterized in that one capacitor (C_(S)) is connected parallel to the second diode (D₂) of the diode series circuit.
 3. The circuit arrangement of claim 1, characterized in that one capacitor (C_(F)) is connected parallel to the first diode (D₁) of the diode series circuit.
 4. The circuit arrangement of claims 1, characterized in that the capacitance parallel to the lamp of the series resonant circuit (16) assigned to each low-pressure discharge lamp (18) comprises two parallel-connected capacitors (C_(R1) and CR_(R2)).
 5. The circuit arrangement of claim 4, characterized in that the following inequality applies to the ratio of the capacitances of the one and another capacitors C_(R1) and C_(R2) parallel to the lamp:

    0.5<C.sub.R1 /C.sub.R2 <2


6. The circuit arrangement of claim 5, characterized in thatthe interference filter (12) is located following the mains rectifier (10); the inductance (L_(K)), the first diode (D₁) and the second diode (D₂) as well as the filter capacitor (C_(S)) are connected in series between the outputs of the interference filter (12); the RF inverter (14) is located between the terminal point between the second diode (D₂) and the filter capacitor (C_(S)) and one output of the interference filter (12); a resistor (R₁) and a capacitor (C₁) are located in series between the terminal point between the second diode (D₂) and the filter capacitor (C_(S)) and one output of the interference filter (12); a bidirectional thyristor diode (DIAC) is connected in series between a terminal point, present between the resistor (R₁) and the capacitor (C₁), and the gate of the second transistor (T₂); the one capacitor (C_(R1)) of the series resonant circuit (16) is connected in parallel to the lamp between the center tap (M) and between the center point (W11) between the first diode (D₁) and the second diode (D₂), and the other capacitor (C_(R2)) is connected to a point located between the second diode (D₂) and the filter capacitor (C_(S)); a capacitor (C_(K)) is connected parallel to the second diode (D₂); the low-pressure discharge lamp or lamps (18) are connected parallel to the first capacitor (C_(R1)); a further resistor (R₃) is connected between the second diode (D₂) and the filter capacitor (C_(S)) and the center tap (M); a resistor (R₂) and a further capacitor (C₂) are connected in series and parallel with the further resistor (R₃); a preheating circuit including two preheating capacitors (C₄, C₅), connected in series, is connected in parallel to the low-pressure discharge lamp (18), and a continuously self-adjusting ohmic resistor (R₄) whose resistance change is dependent on its temperature is connected to the first capacitor (C₄); a third diode (D₃) is connected in series between the first resistor (R₁) and the first capacitor (C₁) and the center tap (M) ; and that the positive terminal of the filter capacitor (C_(S)) is connected to the interference filter (12) via a fourth diode (D₄).
 7. The circuit arrangement of claim 6, characterized in that the following components have the listed values or dimensions:

    ______________________________________                                         First capacitor       C.sub.1 = 100 nF                                                               C.sub.2 = 2.2 nF                                         Coupling capacitor    C.sub.3 = 150 nF                                         Preheater circuit                                                                                     ##STR1##                                                capacitors                                                                                            ##STR2##                                                                      Cπ.sub.1 and Cπ.sub.2 = 150 nF                     Resonance capacitor   CR.sub.1 = 4.7 nF                                        Resonance capacitor   CR.sub.2 = 3.3 nF                                        Filter capacitor      C.sub.5 = 47 μF                                                             C.sub.F = 3.3 nF                                         Capacitor             C.sub.K = 15 nF                                          First diode           D.sub.1 = BA 157                                         Second diode          D.sub.2 = BA 157                                         Third diode           D.sub.3 = 1N4004                                         Fourth diode          D.sub.4 = 1N4004                                                               DIAC = N413                                              First Resistor        R.sub.1 = 330 kΩ                                   Resistor              R.sub.2 = 10 Ω                                     Further resitor       R.sub.3 = 470 kΩ                                                         Lπ = 680 μH                                        Inductance            L.sub.K = 330 μH                                      Resonant inductance   L.sub.3 = 1.4 mH                                         Switching transistors                                                          T.sub.1 and T.sub.2,                                                           Type IRF 224                                                                   ______________________________________                                    


8. The circuit arrangement of claim 6, characterized in that the capacitance of said one capacitor (C_(R1)) of the two capacitors (C_(R1), C_(R2)) and connected effectively parallel to the electrodes (E₁, E₂) is formed by said preheating capacitors (C₄, C₅).
 9. The circuit arrangement of claim 1 characterized in that the coupling capacitor (C₃) is connected according to one of;between the resonant inductance and the junction points of the other capacitor (C_(R2)) parallel to the lamp and the resonant inductance (L₃), or between the junction point of the resonant inductance (L₃) with the other capacitor (C_(R2)) parallel to the lamp and the junction point of the first capacitor (C_(R1)) parallel to the lamp and the first electrode (E₁), or between the junction point of the first and second diode (D₁, D₂) and the one capacitor (C_(R1)) parallel to the lamp and the second electrode (E₂), or between the resonant inductance (L₃) and the junction point of the center tap (M) and the further resistor (R₃). 